Liquid crystals have effective optical properties which can be controlled by an applied electric field. Non-light-emitting elements utilizing the electrooptic effect of liquid crystal are now widely used.
Among liquid crystals, most attention is now paid to ferroelectric liquid crystals which exhibit ferroelectric properties in a smectic C* phase because of fast response and optical bistability or memory effect. The memory effect of ferroelectric liquid crystal is developed by the bistability of liquid crystal molecular alignment.
A variety of methods have been examined for causing liquid crystal to develop optical bistability, including (1) rubbing, (2) shearing, (3) magnetic field application, (4) imparting appropriate orienting nature to the cell side, (5) temperature gradient, and (6) oblique evaporation. Methods (2) to (5) are difficult to apply to elements having practical surface areas. Method (6) is poor in productivity or mass production because the permissible range of optimum evaporation angle is narrow. Therefore, most efforts have been made on modifications of the rubbing method which has been feasible for conventional liquid crystal elements of the TN type as the most productive orienting method.
Most modified rubbing methods require cells as thin as about 1 micron. From the standpoint of manufacturing process, it is very difficult to form such thin cells with a relatively large surface area. As compared with the contrast developed under an applied electric field, the cell provides a low contrast during memory periods when the electric field is removed. As the cell thickness is increased, the bistability is further deteriorated.
AC stabilization is contemplated as reported by J. P. Le Pesant et al., Paris Liquid Crystal Conf., 1984, p 217 and J. M. Geary, SID '85 Digest, p 128. Although AC stabilization achieves good bistability, the resulting liquid crystal element, which is equivalently represented by a parallel circuit of capacitance C and resistance R, requires a high power, losing the benefits of low voltage drive and low power consumption inherent to liquid crystal elements.
In some cases, the necessary drive voltage exceeds the drive voltage range available with conventional liquid crystal element driving IC's. There arises a need for newly developing a special IC for drive, which will undesirably invite a cost increase.
To overcome these problems, Nakaya and Kobayashi proposed a liquid crystal element having an orienting film in the form of a mono-molecular film or built-up film of an organic high molecular weight compound formed by a Langmuir Blodgett (often abbreviated as LB, hereinafter) process (see Japanese Patent Application No. 58005/1988 filed Mar. 11, 1988). This liquid crystal element has improved bistability due to the presence of the orienting film and maintains the bistability even at increased cell thickness in the practical range.
However, further improvements are required for liquid crystal elements not only in bistability, but also in contrast. In addition, the response time that the liquid crystal element of the above proposal actually exhibited did not reach the theoretically presumed value.
In ferroelectric liquid crystal elements, unlike other liquid crystal elements, the spatial divergence of spontaneous polarization that ferroelectric liquid crystal molecules possess, that is, a polarized field created by the associated space charge affects the alignment of liquid crystal molecules (see M. Nakagawa and T. Akahane, J. Phys. Soc. Jpn., 55, 1516 (1986)).
This will be better understood by considering the switching action of a ferroelectric liquid crystal element.
An applied electric field across a ferroelectric liquid crystal causes reversal of spontaneous polarization. The reversed spontaneous polarization creates polarization charge at the interface between the liquid crystal and the orienting film. The polarization charge inhibits reversal of spontaneous polarization, delaying the response time of the liquid crystal element and lowering the contrast available under the applied electric field.
When the electric field is turned off, the charge created at the interface between the liquid crystal and the orienting film during electric field application starts discharging. With this discharging, the spontaneous polarization reversed during electric field application tends to resume the original polarization direction, lowering the contrast during memory periods and failing to provide high bistability.
The ferroelectric liquid crystal inevitably suffers from a lowering of bistability and contrast associated with its spontaneous polarization. A delay of the response time from the theory is also considered due to the spontaneous polarization.
It should be understood that these problems resulting from the spontaneous polarization of ferroelectric liquid crystal arise not only with the orienting film formed by the LB technique, but also in orienting films formed by coating and other techniques.